1. Field of the Invention
The invention relates to a method and to apparatus for the preparation, more particularly for the cooling and mixing of moulding sand, in which said method and apparatus the used sand, obtained from the sand batch is cooled, is cleaned where appropriate, and is mixed with other substances by the addition of water and is homogenized accompanied by intensive agitation and the sand particle surface is at least partially covered with a binding agent.
2. Description of the Prior Art
The purpose of preparing sand in the production of casing moulds is mainly to establish the correct mixing ratio of particle sizes and the ratio of proportions of quartz sand, binder, coal dust as well as old and fresh sand, to homogenize the mixture while covering the particles substantially with binder, to adjust the correct moisture content, to remove unusable constituents, such as sand lumps, iron, dust, to adjust the correct sand temperature and to convey the sand to the places of use. The reused proportion of used sand obtained from the sand batch can amount to 80% and more. Since the sand is obtained from the sand batch at elevated temperatures, for example of 100.degree. C. to 140.degree. C., it is necessary for the used sand to be cooled before being filled into the mixing device. Cooling to temperatures between 35.degree. C. and 45.degree. C. is frequently regarded as adequate. As a rule, the sand is cooled by means of moisture and large quantities of air, blown through the sand, so that the evaporation coldness can be utilized for cooling purposes. The quantities of air required to this end are exceptionally large. In a processing plant adapted to process approximately 80 tons of sand per hour requires air at the rate of several hundred thousand cubic meters per hour and an energy input of up to 160 kwh and more. Apparatus required for cooling the sand occupies a correspondingly large amount of space.
The cooled sand is transferred together with other substances, more particularly fresh sand and binder, into a mixing device. This is usually a batch mixer. Mixing and homogenizing can be performed by means of pug mills with rotating kneading rolls which perform a kneading action on the sand in addition to a mixing action. However, it is more common to employ mixing devices with a mixing vessel containing at the bottom thereof a rotating agitating vane which conveys the material continuously in the radial direction outwardly and upwardly at an angle and additionally contains two or more than two rotating agitating units which extend to a position close above the top edge of the agitating vanes and are adapted to take up and intensively agitate the sand and the constituents. The object of agitation is to homogenize the charge within the shortest possible time, for example 90 seconds, and to cover the surface of the individual sand granules with a binder.
Accurate investigations have shown that complete covering is not achieved. The binder, usually containing clay, takes the form of minute dough-like platelets, which bear only partially on the surface of the sand granules and therefore cover these only partially.
Agitating spiders, extending directly to the agitating vanes which rotate on the bottom and are driven at speeds of 1500 rev/min and more are used to agitate the charge.